Some endpoints are placing backup data on the wrong drive on the RBackup Server

All of these endpoints probably have their Root Directory set to drive C:, OR they used to have it set to drvie C: and their root directories were subsequently changed without moving their data.

Open the RBS Manager (the console utility, not the web console)

Click the Accounts button.

Double click each account to open its Edit screen.

Select a new Root Directory for each of these endpoins. Answer YES when asked if you want to move their data.

If the Root Directories are already set to a drive other than C: then you can delete the old data on drive C: because they are sending their new data to the other drive. If you need to save the old data you can merge it with the new data by copying it to the other drive without overwriting folders.